[DocsUI Peek] Define entry point for showing preview

`ShowPreview` is the entry point for showing a preview, either
via the inspector activity or the new peek capability, depending
on the flags state.

Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.use_peek_preview
Test: atest
DocumentsUIGoogleTests:com.android.documentsui.files.ActionHandlerTest
Tests the right preview action depending on the flags state.
Change-Id: Ief227bc8729d1115c2f4fe0a55a72b846645ec5d
Bug: 388390364